1. Field of the Invention
The present invention relates to a document feeding apparatus and a document feeding method used in a copying machine for feeding a plurality of document sheets sequentially to a document scanning area where the documents are scanned for exposure.
2. Description of the Prior Art
It is well known in the prior art to provide copying machines with automatic document feeding apparatus for automatically feeding a plurality of document sheets in sequential fashion to a document scanning area thereby alleviating the work of the operator. Images of the documents fed one by one to the document scanning area by the document feeding apparatus are scanned for exposure, thus accomplishing simplex (single-sided) or duplex (two-sided) copying in cooperation with the copying machine.
FIG. 1 is a cross sectional view schematically showing the structure of a document feeding apparatus 1 in a typical prior art example. The document feeding apparatus 1 is mounted, for example, on the top surface 3 of an electrostatic copying machine 2. A transparent plate 4 providing a document scanning area is fitted into the top surface 3. Documents d are fed in a transporting direction 5 and on to the transparent plate 4.
The document feeding apparatus 1 includes an endless belt 8 which are passed around a pair of rollers 6 and 7 disposed spaced apart along the transporting direction 5. On the inside of the endless belt 8 and adjacent to the lower taut portion thereof facing the transparent plate 4, there are disposed a plurality of pressure rollers 9a-9e in this order along the document transporting direction 5. The documents d stacked on a document loading tray 10 are fed one by one from the top of the stack in the transporting direction 5 by a document feeding means not shown, and the document d thus fed is transported along the upper surface of the transparent plate 4 by means of the belt 8 rotating in the clockwise direction (in FIG. 1) until the document d reaches the scanning position with its leading edge hitting a stop member 11. At this time, the pressure rollers 9a-9e apply pressure to press the document d against the transparent plate 4 via the belt 8 so as to prevent the document d being transported to the scanning position from lifting.
The document d transported to the scanning position is then scanned for exposure of the document image by moving an optical scanning means 14, including a light source 12, a reflecting mirror 13, etc., in the direction shown by arrow 15. After the document d has been scanned at the scanning position, a solenoid not shown is actuated to release the stop member 11 from the document d and the belt 8 are restarted for rotation to discharge the document d through a transport path 16 onto a document exit tray 17. The above feeding operation is sequentially performed on the documents d stacked on the document loading tray 10.
In the above construction that sequentially feeds a plurality of sheet documents to the scanning position, the time it takes to feed the document subsequent to the previously fed one greatly affects the time it takes for the whole copying operation.
FIG. 2 shows diagrams explaining problems with the prior art construction. A document to be fed first is designed by reference numeral d1 and a document to be fed subsequently to the first one is designated by d2. When the document size is to be designated, a suffix L (meaning large size) or S (meaning small size) is attached to the same reference numerals. The document d1 fed first is transported by means of the belt 8 until reaching the scanning position where it contacts the stop member 11. When the document d1 has reached the scanning position, the rotation of the belt 8 stops and the document d1 is scanned by the optical scanning means 14. While the document d1 is being scanned, the succeeding document d2 is transported to a halfway position on the transparent plate 4.
In the above prior art construction, the pressure against the belt 8 of the pressure rollers 9a-9e are not individually controlled. In other words, all the pressure rollers 9a-9e are pressed against the transparent plate 4 via the belt 8, so as to prevent the document at the scanning position from lifting. As a result, on the downstream side from the upstream end pressure roller 9a with respect to the transporting direction, the belt 8 and the transparent plate 4 are pressed against each other with the document d1 interposed therebetween. Since the belt 8 are not rotating during the scanning period, the document d2 is subjected only to the transporting force by the document feeding means and cannot be made to enter the space between the belt 8 and the transparent plate 4 on the downstream side of the pressure roller 9a in the transporting direction. That is, during the scanning of the leading document d1, the succeeding document d2 can only be fed to a standby position where the leading edge thereof reaches the upstream end pressure roller 9a. Feeding the document d2 further downstream of that position is not possible since it would cause the leading edge thereof to be bent or jammed.
FIG. 2(1) shows the condition in which a document d1L having a length La1 along the transporting direction 5 has been transported to the scanning position. During the scanning of the document d1L, the succeeding document d2L can be transported to the position of the pressure roller 9a to stand by at that position. In other words, the succeeding document d2L can be preliminarily transported on the transparent plate 4 by a distance Lc1 and d2L. This means that when the rotation of the belt 8 is restarted after completing the scanning of the document d1L, the succeeding document d2L is transported by the length La1 plus the distance Lb1 to reach the scanning position.
FIG. 2(2) shows the condition in which a document d1S having a length La2 along the transporting direction 5 has been transported to the scanning position. In this case, the following relationship holds. EQU La1&gt;La2 (1)
In the prior art construction, even in the case of the document d2S with a smaller size, the succeeding document d2S can only be transported to the position of the pressure roller 9a for standby as described above. Therefore, the distance Le2 over which the document d2S is preliminarily transported on the transparent plate 4 is expressed by the following relation. EQU Lc1=Lc2 (2)
Therefore, the distance Lb2 between the documents d1S and d2S has the following relationship with respect to the distance Lb1 between the larger sized documents d1L and d2L. EQU Lb1&lt;Lb2 (3)
However, the distance over which the succeeding document d2S is transported to the scanning position after completing the scanning of the leading document d1S is expressed as the length La2+the distance Lb2, which is equal to the distance over which the larger sized document d2L is transported to the same position. That is, since the standby position of the succeeding document d2 is the same despite the difference in size, the time needed to transport a plurality of document sequentially to the scanning position is the same regardless of the document size.
As is apparent from FIG. 2, the distance between the documents transported becomes larger as the size of the document d becomes smaller. If the distance can be reduced, the time to transport the succeeding document d2 to the scanning position following the preceding document d1 can be shortened, which can contribute to a significant reduction in the time needed to transport a plurality of documents sequentially to the scanning position.
As described earlier, increasing numbers of copying machines are being provided with automatic document feeding apparatus for automatically feeding a plurality of sheet documents in sequential fashion to a document scanning area thereby alleviating the work of the operator. The documents to be copied are stacked on a document loading tray and are fed sequentially from the bottom of the stack, for example. The documents are transported by means of transport belts sequentially to the document scanning area with the document image to be copied facing the interior side of the copying machine, and the thus positioned document image is scanned by an optical scanning means, for example, thus accomplishing simplex (single-sided) or duplex (two-sided) copying in cooperation with the copying machine.
In order to transport and position the document first with one side thereof facing the document scanning area and then with the other side thereof facing it, there is provided, for example, an inverting means between the document loading tray and the document scanning area. The inverting means consists of a first transport path and a second transport path provided between the document loading tray and the document scanning area. For simplex (single-sided) copying, the document fed from the document loading tray is turned over by passing through the first transport path and is then transported with the image side thereof facing the document scanning area. On the other hand, for duplex (two-sided) copying, the document fed from the document loading tray is first transported into the first transport path where the transport direction is reversed and is then directed into the second transport path, after which the document is transported to the document scanning area. As a result, the document is positioned with the reverse thereof facing the document scanning area without having to turn back the document placed on the document loading tray. After the reverse side has been scanned, the document is returned with its sides inverted, after which the document is refed through the first and second transport paths, thereby allowing the remaining side, i.e. the top side, of the document to be scanned for exposure.
As described, in the construction in which a plurality of document sheets are sequentially fed to the scanning area, the time it takes to feed a next document subsequent to the preceding document greatly affects the time the whole copying operation takes. Therefore, there can be considered a construction in which the succeeding document is preliminarily fed to a standby position as close as possible to the preceding document while the preceding document stationary on the document scanning area is being scanned by the optical scanning means, thereby shortening the time needed to transport a plurality of documents sequentially to the document scanning area. The construction in which the succeeding document is preliminarily fed to a standby position as described above is generally known as preliminary feeding, an example of which is disclosed in Japan Laid Open No. 62-12533. According to the construction disclosed therein, the succeeding document preliminarily fed is placed on standby immediately before the transparent plate that provides the document scanning area.
In preliminary feeding for duplex (two-sided) copying, for example, the document fed from the document loading tray must first be passed through the first transport path and inverted before being passed through the second transport path, as described above. Therefore, in order to accomplish the preliminary feeding in the case of a document having a relatively large dimension along the transporting direction, the first transport path must have a sufficient length that can accommodate that dimension. This tends to increase the size of the inverting means. In other words, if the inverting means is reduced in size, preliminary feeding cannot be performed in the case of a larger sized document. On the other hand, increasing the path length by enlarging the inverting means results in an increase in the transporting time of the document. The increase in the transporting time is particularly appreciable in the case of simplex(single-side) copying in which the document is transported only through the first transport path. Thus, there is a limit to the reduction of the time needed to sequentially transport a plurality of documents, which presents an obstacle to the reduction of the copying time.